Optical disk apparatus employing high-NA objective lens

ABSTRACT

Protecting member is provided on lens-holding member to protrude toward optical disk more than objective lens. Further protecting member is prevented from contact with cover layer, even when movable part is not driven by actuator and elastic supporting members are bent by gravity acting on movable part toward optical disk.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a portable optical disk apparatus forrecording or reproducing information signals on an optical diskemploying an objective lens of NA (numerical aperture) of 0.8 or higher.

2. Related Background Art

Optical disk apparatuses are commercialized which record informationsignals by focusing a laser beam in a fine spot by an objective lens onan information signal recording layer of an optical disk or reproducingthe recorded information signals by the reflected light. Lately, it hasbecome possible to increase NA of the objective lens of the optical diskapparatus to 0.8 or higher as the results of improvement in design andproduction technique of the objective lens. Such a high-NA objectivelens enables formation of a finer light spot to improve resolution ininformation recording and reproducion and to increase the capacity ofthe optical disk.

Conventionally, a usual optical disk has an information signal recordinglayer formed on a substrate plate of 0.6-1.2 mm thick composed of atransparent resin material. The optical disk apparatus projects a laserbeam through the substrate plate onto the information signal recordinglayer. However, an objective lens of a higher NA has a shorter focuslength, requiring a shorter distance between the objective lens and theinformation signal recording layer. This makes it difficult to focus alaser beam through the substrate plate of 0.6-1.2 mm thick on theinformation signal recording layer. To solve this problem, methods aredisclosed. In one method, to meet increase of NA of the objective lens,the information signal recording layer is covered on the face reverse tothe substrate plate side with a transparent cover layer thinner than thesubstrate plate, and the laser beam is projected through the coverlayer, not through the substrate plate, as disclosed in Japanese PatentApplication Laid-Open Nos. 2003-91833, 2004-30835, and so forth.

FIG. 4 illustrates schematically a constitution of such an optical diskapparatus of a prior art. In FIG. 4, the numeral 1 denotes an opticaldisk, the numeral 2 denotes an optical head, and the numeral 3 denotes aspindle motor to rotate the optical disk. Optical head 2 is constitutedof laser light source 6, collimator lens 7, beam splitter 8, condenserlens 9, photodetector 10, objective lens 20, actuator 11 for controllingthe focusing and tracking with the objective lens 20.

Optical disk 1 is constituted of substrate plate 12, information signalrecording layer 13 formed on substrate plate 12 from a phase changeablematerial capable of changing reversibly the phase state, and cover layer14 of about 0.1 mm thick composed of a transparent resin material. Oninformation signal recording layer 13, recording tracks are formed in aspiral or concentric circles. The information signal recording layer 13may be formed from a magnetooptical material, or a metallic reflectingfilm having pits (concaves) formed thereon. Optical head 2 is placed toface cover layer 14 of optical disk 1.

FIG. 5 shows constitution of actuator 11. Actuator 11 comprisesimmovable part 15 and movable part 16. Immovable part 15 is constitutedof permanent magnets 17 a,17 b, yoke 18, and supporting stage 19.Movable part 16 is constituted of objective lens 20, focusing coil 21,tracking coil 22, and lens-holding member 23 for holding the abovearticles. Elastic supporting members 24 a,24 b,24 c,24 d are in a linearshape, and are elastic and highly electroconductive. The supportingmembers are fixed at the respective ends to supporting stage 19, andhold, at the other ends, movable part 16 to be movable freely invertical and radial directions relative to optical disk 1. Elasticsupporting members 24 a,24 b,24 c,24 d are also connected electricallyto focusing coil 21 and tracking coil 22 on lens-holding member 23.

The optical disk apparatus, as shown in FIG. 4, has error signalgenerating circuit 4 and control circuit 5 for control of focusing andtracking by actuator 11. Control circuit 5 applies electric controlcurrent through elastic supporting members 24 a,24 b,24 c,24 d tofocusing coil 21 and tracking coil 22.

In recording the information signals, firstly optical disk 1 is rotatedby spindle motor 3. With the optical disk kept rotated, a laser beampulse-modulated in accordance with information signals and emitted fromby laser light source 6 is parallelized by collimator lens 7, allowed topass beam splitter 8, and focused in a fine spot by objective lens 20through cover layer 14 of optical disk 1 on information signal recordinglayer 13.

Information signal recording layer 13 of optical disk 1 is heated andcooled repeatedly by projection of a pulse-modulated laser beam to formrecording marks by phase change to an amorphous phase or a crystal phaseby the heating-cooling process.

In reproduction of the recorded information signals, optical disk 1 isrotated similarly by spindle motor 3. With the optical disk keptrotated, a constant intensity of a laser beam emitted from laser lightsource 6 is focused in a fine spot through cover layer 14 on informationsignal recording layer 13. The intensity of the light reflected frominformation signal recording layer 13 varies in accordance with therecorded marks, whereby the information signal is reproduced.

During the operation of recording and reproducing the informationsignals, the light beam reflected by optical disk 1 is further reflectedby beam splitter 8, condensed by condenser lens 9, and detected byphotodetector 10. Photodetector 10 has a light-receiving face dividedinto plural sections. Error signal generating circuit 4 generates focuserror signals and tracking error signals according to the detectedsignals at the respective sections of the light-receiving face.

Control circuit 5 applies a control current based on the focus errorsignal and the tracking error signal through elastic supporting members24 a, 24 b, 24 c, 24 d to focusing coil 21 and tracking coil 22.Actuator 11 drives movable part 16 to move perpendicularly toward oraway from optical disk 1 or in the radial directions crossing therecording track by an electromagnetic force generated by interaction ofthe control current and the magnetic fluxes of permanent magnets 17 a,17b.

In such a manner, the focusing is controlled to focus precisely thelight spot on the recording track to offset perpendicular displacementby axial deflection of surface of optical disk 1 or a like cause, andthe tracking is controlled to scan the recording track by offsettingradial run-out of the track center in the radial direction.

In the above optical disk apparatus, since the gap (working distance)between objective lens 20 and the surface of cover layer 14 is as smallas 0.2-0.6 mm, a stopper or a like mechanism cannot be provided at aposition in the gap to limit the displacement of movable part 16 toprevent undesired contact of objective lens 20 with optical disk 1.Naturally, the actuator normally working for the control will keepsconstant the gap between objective lens 20 and the surface of coverlayer 14. However, actuator may fail in the control owing to abruptimpact, vibration, or a like cause, or adverse effect of a dirt orscratch on optical disk 1 not to give a normal error signal.

In such an abnormal working state, movable part 16 is excessivelydisplaced inevitably to come into contact with optical disk 1. Even whenthe contact occurs, at least objective lens 20 can be protected byproviding a protrusion higher than objective lens 20 at a portion oflens-holding member 23 facing to optical disk 1 and allowing theprotrusion to touch optical disk 1.

However, even in a non-working state, especially with the power sourceturned off, when optical disk apparatus is placed with optical disk 1held horizontally and with optical head 2 held above the disk, elasticsupporting members 24 a,24 b,24 c,24 d may be bent by gravity acting onmovable part 16 to cause contact of movable part 16 with optical disk 1.This contact state can sometimes be kept for a long time by standingleft.

Usually, lens-holding member 23 and cover layer 14 of optical disk 1 aremade from different kinds of resins. The different kinds of resins whichare kept in contact with each other under pressure are liable to undergonot only usual environmental deterioration but also a physical changesuch as local deformation and a chemical change such as chemicaldeterioration. In particular, the protecting member should haveessential properties for productivity (formability) and mechanicalproperties such as rigidity, and the cover layer should have opticalproperties and physical properties such as hardness. Therefore physicaland chemical durability cannot readily be given additionally to theprotecting member or the cover layer. The deterioration of opticalproperties of cover layer 14 caused by deformation or deterioration ofthe contact portion can presumably prevent normal recording andreproduction of the information signals disadvantageously.

In recent years, optical disk apparatuses are coming to be used as aportable apparatus for recording and reproducing voices, still-pictures,animated pictures, and so forth. The portable apparatuses are storedarbitrarily in various manners by users, stored in many cases with theoptical disk kept inserted in the apparatus. Accordingly, the opticaldisk apparatus having the objective lens of a high NA will inevitablyencounter the above-mentioned problems.

SUMMARY OF THE INVENTION

The present invention intends to provide an optical disk apparatus whichdoes not cause undesired contact of an objective lens with optical diskregardless of the placement state of the optical disk apparatus.

According to an aspect of the present invention, there is provided anoptical disk apparatus constituted of a movable part having an objectivelens for focusing a laser beam onto an optical disk, a lens-holdingmember for holding the objective lens, and a protecting member forpreventing contact of the objective lens with the optical disk; anelastic supporting member for supporting the movable part; and anactuator for driving the movable part toward or apart from the opticaldisk,

wherein the apparatus is constructed not to cause contact of theprotecting member with the optical disk in a non-working state of theactuator even when the elastic supporting member is bent by gravitytoward the optical disk.

The optical disk preferably has a recording layer and a transparentcover layer formed on the recording layer, and the protecting memberprevents contact of the objective lens with the cover layer.

The apparatus is preferably constituted to satisfy the condition below:S−S _(v) +S ₀ −A>0where S denotes a distance between the optical disk and the protectingmember with the actuator working, S_(v) denotes the maximum of decreaseof the distance between the optical disk and the protecting member byvariable factors, S₀ denotes a withdrawal distance with the actuatornon-working, and A denotes displacement of the protecting member bygravity acting on the movable part.

The optical disk apparatus is preferably portable. The objective lenspreferably has an NA not lower than 0.8. (The statements correspondingto claims are omitted.)

According to the present invention, the moving part will not come intocontact with the optical disk by bending by the gravity (the weight ofthe movable part) of the elastic supporting member for supporting themovable part toward the optical disk, even when the optical disk isplaced during a non-working state, in particular with the power sourceturned off, with the optical disk kept horizontally and with the opticalhead kept above the optical disk. Therefore, even if the apparatus isstored for a long time in the above-mentioned state, the contact of themovable part with the optical disk is prevented not to impair thereliability of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates constitution of an actuator of an optical diskapparatus of the present invention.

FIG. 2 is a drawing for explaining working of the optical disk of thepresent invention placed in a vertical direction.

FIG. 3 is a drawing for explaining working of the optical disk of thepresent invention placed in a horizontal direction.

FIG. 4 illustrates schematically constitution of an optical diskapparatus.

FIG. 5 illustrates constitution of an actuator of a conventional opticaldisk apparatus.

FIG. 6 is a drawing for explaining working of optical disk of thepresent invention placed in another horizontal direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The best mode for carrying out the invention is explained below indetail by reference to drawings. The constitution and operation of theoptical disk apparatus of the present invention is similar to theconventional one shown in FIG. 4, so that detailed explanation thereofis omitted. The present invention relates to optical disk apparatuses,especially to portable ones, for recording or reproduction of voices,still-pictures, animated pictures, or the like, and assumes the use andstorage in an arbitrary placement direction of the apparatus.

Actuator 11 of the optical disk apparatus of the present invention isexplained below in detail. FIG. 1 shows constitution of actuator 11. InFIG. 1, the same symbols as in FIG. 5 are used for denoting thecorresponding members. Actuator 11 comprises immovable part 15 andmovable part 16. Immovable part 15 is constituted of permanent magnets17 a, 17 b, yoke 18, and supporting stage 19. Movable part 16 isconstituted of objective lens 20 of an NA of 0.8 or higher, focusingcoil 21, tracking coil 22, and lens-holding member 23 for holding theabove articles. Lens-holding member 23 has protecting member 25protruding a little higher than objective lens 20 toward the opticaldisk 1. Protecting member 25 may be formed in integration withlens-holding member 23, or may be formed separately and attached laterthereto.

Elastic supporting members 24 a, 24 b, 24 c, 24 d are in a linear shape,and are elastic and highly electroconductive. The supporting members arefixed at the respective ends to supporting stage 19, and hold, at theother ends, movable part 16 to be movable freely in vertical and radialdirections relative to optical disk 1. Elastic supporting members 24 a,24 b, 24 c, 24 d are also connected electrically to focusing coil 21 andtracking coil 22 on lens-holding member 23.

The optical disk apparatus, as shown in FIG. 4, has error-signalgenerating circuit 4 and control circuit 5 for focusing and tracking byactuator 11. Control circuit 5 applies an electric current for controlthrough elastic supporting members 24 a, 24 b, 24 c, 24 d to focusingcoil 21 and tracking coil 22.

In the above optical disk apparatus, since the gap (working distance)between objective lens 20 and the surface of cover layer 14 is as smallas 0.2-0.6 mm, a stopper or a like mechanism cannot be provided at aposition in the gap to limit the displacement of movable part 16 toprevent undesired contact of objective lens 20 with cover layer 14.Naturally, the actuator normally working for the control will keepconstant the gap between objective lens 20 and the surface of coverlayer 14. However, actuator may fail in the control owing to abruptimpact, vibration, or a like cause, or owing to adverse effect of a dirtor scratch on optical disk 1 not to give a normal error signal.

When movable part 16 comes to be displaced excessively toward opticaldisk 1 in such an abnormal working state, protecting member 25protruding higher than objective lens 20 is brought into contact withcover layer 14 of optical disk 1 to protect objective lens 20 and toprevent scratch formation on cover layer 14. To prevent a seriousadverse effect on reliability of optical head 2 or optical disk 1,protecting member 25 is formed from a slidable material and cover layer14 is formed from a hard material; and immediately after detection ofabnormal working, movable part 16 is withdrawn once away from opticaldisk 1 and later the control working is restarted. Thereby the contactbetween protecting member 25 and optical disk 1 is limited to beinstantaneous, not affecting seriously the reliability of optical head 2and optical disk 1.

FIG. 2 is a side view of actuator 11 and optical disk 1 of an opticaldisk apparatus of the present invention with optical disk setvertically. In this placement direction, the gravity acts parallel toelastic supporting members 24 a, 24 b, 24 c, 24 d as indicated by arrowmark G, so that the elastic supporting members 24 a, 24 b, 24 c, 24 dare not bent by the gravity acting on movable part 16, namely the weightthereof. This placement state in which the elastic supporting membersare not bent by the gravity is hereinafter referred to as“non-gravitated placement state”. When activator 11 is not working(hereinafter referred to as “neutral state”) in the non-gravitatedplacement state, for example the power source of the apparatus is turnedoff, elastic supporting members 24 a,24 b,24 c,24 d keep the position asshown by solid lines 51 without elastic deformation. Here, the positionof the top of protecting member 25 in the neutral state is denoted by asymbol P′. By operation of actuator 11 for focusing control, elasticsupporting members 24 a,24 b,24 c,24 d are elastically deformed as shownby one-dot chain lines 53 to displace movable part 16 toward opticaldisk 1 by distance S₀ from the neutral state. The focus-controlled stateby actuator 11 is hereinafter referred to as “controlled state”. Theposition of the top of protecting member 25 in the controlled state isdenoted by a symbol P. The distance between position P′ of theprotecting member in the neutral state and position P thereof in the inthe controlled state is referred to as “withdrawal distance” ofprotecting member 25. In the controlled state, the distance betweenposition P of the top of protecting member 25 and cover layer 14(hereinafter referred to as “non-contact interval”) is kept invariablyat a constant interval S. Thus, movable part 16 is installed such thatposition P′ of the top of protecting member is apart from optical disk 1more than position P thereof at the neutral state by withdrawal distanceS₀. Therefore, in the non-gravitated placement in the neutral state, theinterval L between the top of protecting member 25 and cover layer 14(hereinafter referred to as a “neutral state interval”) is a sum of thenon-contact interval S and withdrawal distance S₀:L=S+S ₀

FIGS. 3 and 6 are side views of actuator 11 and optical disk 1 of anoptical disk apparatus of the present invention with optical disk 1placed horizontally and optical head 2 placed above the disk,respectively. In this placement direction, the gravity actsperpendicularly on elastic supporting members 24 a, 24 b, 24 b, and 24 das indicated by arrow mark G. In this placement direction, in a neutralstate, elastic supporting members 24 a, 24 b, 24 c, 24 d are bent by thegravity acting on movable part 16, namely the weight thereof, towardoptical disk 1 from the position in the non-gravitated placement in theneutral state as shown by one-dot chain line 51 toward optical disk 1 asshown by solid line 52. The placement state of the optical diskapparatus in which the elastic supporting member is bent by the gravityis referred to as a “gravitated placement state”. The position of thetop of protecting member 25 in the neutral state in the gravitatedplacement state is indicated by a symbol P″. In this gravitatedplacement state, by focusing control by actuator 11, movable part 16 isdriven to cause elastic deformation of 24 a, 24 b, 24 c, and 24 d asshown by one-dot chain lines and continuous line 51 to 53 in FIGS. 3 and6 to keep constant the non-contact interval S between position P of thetop of protecting member 25 and cover layer 14. The degree of bending(hereinafter referred to as a “displacement distance”) of protectingmember 25 by the gravity from the position P′ in the neutral andnon-gravitated placement state is indicated by a symbol A. Then theneutral state distance D in the neutral state in the gravitatedplacement state is equal to non-contact interval S plus withdrawaldistance S₀ minus displacement distance A:D=S+S ₀ −A

Actually, in any placement state, in the neutral state, the relativeinterval between the top of protecting member 25 and cover layer 14 isvariable around the designed dimension owing to warpage and thicknesserror of optical disk 1, variation of setting position of the opticalhead, or other causes. In consideration of a possible maximum deviationS_(v) toward the optical disk (or the cover layer) by variable factors,the minimum value L_(min) Of the neutral state interval L in thenon-gravitated placement state is represented by the equation below:L _(min) =S−S _(v) +S ₀and the minimum value D_(min) of the neutral state interval in thegravitated condition is represented by the equation below:D _(min) =S−S _(v) +S ₀ −AIn the case where the deviation A is larger than the minimum(L_(min)=S−S_(v)+S₀) at the neutral state under non-gravitatedcondition, the protecting member 25 is kept in contact with cover layer14. Therefore, to prevent the contact of protecting member 25 with coverlayer 14, L_(min) should be larger than A:D _(min) =S−S _(v) +S ₀ −A>0  (1)

The non-contact distance S in the controlled state is necessarily shortas the result of shortening of the focal distance resulting fromincrease of NA of objective lens 20. On the other hand, the decreaseS_(v) of the interval results from variation in production of theoptical disk and apparatus and other variable factors, so that thereduction thereof is limited. Therefore, in the present invention,withdrawal distance S₀ and displacement distance A are suitably designedto satisfy the above equation (1). The above matter is explained morespecifically by reference to Examples.

The withdrawal of movable part 16 in the neutral state is conductedwithout employing a driving means requiring electricity. Therefore themovable part 16 is withdrawn surely in a power turn-off state of theoptical apparatus.

The above matter is explained below more specifically by reference toExamples.

EXAMPLE 1

In the apparatus of this Example, the laser beam has a wavelength of 405nm; objective lens 20 has an NA of 0.85; cover layer 14 of optical disk1 has a thickness of 0.1 mm; the interval between the surface of coverlayer 14 and objective lens 20 (working distance) is 0.3 mm at focusingof the laser beam on information signal recording layer 13; protectingmember 25 protrudes higher than objective lens 20 by 0.05 mm; and theinterval between position P of the top of protecting member 25 and coverlayer 14, namely non-contact interval S, is 0.25 mm during focusingcontrol with the actuator, namely in the controlled state.

Movable part 16 of optical head 2 has a mass of 270 mg. The total of thespring constants of elastic supporting members 24 a, 24 b, 24 c, 24 d is19 N/m. Displacement distance A is 0.14 mm with optical disk 1 placedhorizontally.

At the neutral state, the maximum deviation S_(v) to cause decrease ofthe relative interval between the top of protecting member 25 and coverlayer 14 by various variation factors is 0.2 mm.

In this state, if the position of movable part 16 in the neutral stateis the same as the controlled position, namely S₀=0, the minimum D_(min)of the interval between protecting member 25 and cover layer 14 iscalculated as below: $\begin{matrix}{D_{\min} = {S - S_{v} + S_{0} - A}} \\{= {0.25 - 0.2 + 0 - 0.14}} \\{= {{- 0.09}\quad({mm})}}\end{matrix}$Therefore, the protecting member actually comes into contact with thecover layer.

In this Example, the protecting member is withdrawn by a withdrawaldistance S₀ of 0.1 mm. Then the minimum D_(min) of the interval betweenprotecting member 25 and cover layer 14 is calculated as below:$\begin{matrix}{D_{\min} = {S - S_{v} + S_{0} - A}} \\{= {0.25 - 0.2 + 0.1 - 0.14}} \\{= {0.01\quad({mm})}}\end{matrix}$Thus the relation of the above Equation (1) is satisfied. As the result,even when elastic supporting members 24 a,24 b,24 c,24 d are keptbending toward optical disk 1 by gravity of movable part 16, protectingmember 25 does not come into contact with cover layer 14.

EXAMPLE 2

In this Example also, cover layer 14 of optical disk 1 has a thicknessof 0.1 mm; the laser beam has a wavelength of 405 nm; objective lens 20has an NA of 0.85; the interval between the surface of cover layer 14and objective lens 20 (working distance) is 0.3 mm at focusing of thelaser beam on information signal recording layer 13; protecting member25 protrudes more than objective lens by 0.05 mm; and non-contactinterval S is 0.25 mm in the controlled state.

Movable part 16 of optical head 2 has a mass of 230 mg, which is lessthan that in Example 1. The total of the spring constants of elasticsupporting members 24 a, 24 b, 24 c, and 24 d is 56 N/m, which is morethan that in Example 1. Displacement distance A is decreased to 0.04 mmwith the optical disk placed horizontally.

At the neutral state, the maximum deviation S_(v) to cause decrease ofthe relative interval between the top of protecting member 25 and coverlayer 14 by various variation factors is 0.2 mm.

The decrease of the mass of movable part 16 and the increase of thespring constant of elastic supporting member 24 a,24 b,24 c,24 d enablessufficient decrease of the displacement distance A. Thereby, withoutwithdrawal of movable part 16 in the neutral state from the controlledposition (S₀=0), the minimum D_(min) of the interval between protectingmember 25 and cover layer 14 is calculated as below: $\begin{matrix}{D_{\min} = {S - S_{v} + S_{0} - A}} \\{= {0.25 - 0.2 + 0 - 0.04}} \\{= {0.01\quad({mm})}}\end{matrix}$Thus the relation of the above Equation (1) is satisfied. As the result,even when elastic supporting members 24 a, 24 b, 24 c, and 24 d are benttoward optical disk 1 by gravity acting on movable part 16, theprotecting member 25 does not come into contact with cover layer 14.

In the above embodiments explained above, protecting member 25 is formedas a part of lens-holding member 23, but is not limited thereto.Protecting member 25 may be formed at any position of movable part 16driven together with objective lens 20, or on objective lens 20 itself.For example, protecting member 25 may be formed from a transparent glassor a plastic material to protrude from objective lens 20 in integration.In this case, protecting member 25 has preferably a surface filmconstituted of a material having high slidability not to cause damage ofobjective lens 20 and cover layer on contact with optical disk 1.

This application claims priority from Japanese Patent Application No.2004-183721 filed on Jun. 22, 2004, which is hereby incorporated byreference herein.

1. An optical disk apparatus constituted of a movable part having anobjective lens for focusing a laser beam onto an optical disk, alens-holding member for holding the objective lens, and a protectingmember for preventing contact of the objective lens with the opticaldisk; an elastic supporting member for supporting the movable part; andan actuator for driving the movable part toward or apart from theoptical disk, wherein the apparatus is constructed not to cause contactof the protecting member with the optical disk in a non-working state ofthe actuator even when the elastic supporting member is bent by gravitytoward the optical disk.
 2. The optical disk apparatus according toclaim 1, wherein the optical disk has a recording layer and atransparent cover layer formed on the recording layer, and theprotecting member prevents contact of the objective lens with the coverlayer.
 3. The optical disk apparatus according to claim 1, wherein theapparatus is constituted to satisfy the condition below:S−S _(v) +S ₀ −A>0 where S denotes a distance between the optical diskand the protecting member with the actuator working, S_(v) denotes themaximum of decrease of the distance between the optical disk and theprotecting member by variable factors, S₀ denotes a withdrawal distancewith the actuator non-working, and A denotes displacement of theprotecting member by gravity acting on the movable part.
 4. The opticaldisk apparatus according to claim 1, wherein the optical disk apparatusis portable.
 5. The optical disk apparatus according to claim 4, whereinthe objective lens has an NA not lower than 0.8.